Variable-mechanical-impedance artificial legs

ABSTRACT

In one aspect, the invention provides methods and apparatus facilitating an adjustable-stiffness prosthesis or orthosis (including approximations to arbitrarily definable non-linear spring functions). Spring rates may be varied under no-load conditions during a walking gate cycle to minimize power consumption. In another aspect, the invention provides methods and apparatus for outputting positive power from a prosthesis or orthosis, facilitating high-performance artificial limbs. In one embodiment of the invention, the positive power is transferred from a functioning muscle to the prosthesis or orthosis, which mimics or assists a non-functioning or impaired muscle. In another embodiment of the invention, the positive power comes from an on-board power source in the prosthesis or orthosis.

This patent application is a continuation of U.S. patent applicationSer. No. 10/613,499 filed Jul. 3, 2003, entitled“Variable-Mechanical-Impedance Artificial Legs” which claims priority toand the benefit of U.S. Provisional Patent Application No. 60/395,938,filed Jul. 15, 2002, entitled “Variable-Mechanical-Impedance ArtificialLegs” the entire contents of each of which is hereby incorporated byreference in their entirety.

The invention relates generally to the fields of legged robotics,orthotic leg devices and prosthetic leg joints, and more specifically toartificial limbs with time-variable mechanical parameters.

BACKGROUND

Prosthetic limbs have come a long way since the days of simple wooden“peg legs”. Today, amputee men running on a prosthetic leg can beat racetimes of the best unimpaired women runners. It is believed that newadvances in prosthetic limbs (such as those embodied in the presentinvention) will soon lead to amputees being able to out-perform the bestunimpaired athletes of the same sex in sports such as running. It is anobject of the present invention to advance the state of prosthetic limbsto a new level, providing increased athletic performance, increasedcontrol, and reduced body strain. It is a further object of the presentinvention to provide essential elements needed for making prostheticlimbs that more accurately mimic the mechanical behavior of healthyhuman limbs.

Description of Normal, Level-Ground Walking:

In order to establish terminology used in this document, the basicwalking progression from heel strike to toe off is first explained.There are three distinct phases to a walking stance-period as depictedin FIG. 1 with heel-toe sequence 1 through 7.

Saggital Plane Knee Phases

-   -   1. Beginning with heel strike, the stance knee begins to flex        slightly (Sequence 1-3). This flexion allows for shock        absorption upon impact as well as keeping the body's center of        gravity at a more constant vertical level throughout stance.    -   2. After maximum flexion is reached in the stance knee, the        joint begins to extend again, until full extension is reached        (Sequence 3-5).    -   3. During late stance, the knee of the supporting leg begins to        flex again in preparation for the swing phase (Sequence 5-7).        This is referred to in the literature as ‘knee break’. At this        time, the adjacent foot strikes the ground and the body is in        “double support mode” (that is to say, both legs are supporting        body weight).        Saggital Plane Ankle Phases    -   1. Beginning with heel strike, the ankle undergoes a controlled        plantar-flexion phase where the foot rotates towards the ground        until the forefoot makes contact (Sequence 1-2).    -   2. After controlled plantar-flexion, the ankle undergoes a        controlled dorsi-flexion phase where the tibia rotates forwardly        while the foot remains in contact with the ground (Sequence        2-5).    -   3. During late stance, the ankle undergoes a powered        plantar-flexion phase where the forefoot presses against the        ground raising the heel from the ground (Sequence 5-7). This        final phase of walking delivers a maximal level of mechanical        power to the walking step to slow the fall of the body prior to        heel strike of the adjacent, forwardly positioned leg.

The development of artificial leg systems that exhibit natural knee andankle movements has been a long standing goal for designers of leggedrobots, prostheses and orthoses. In recent years, significant progresshas been made in this area. The current state-of-the-art in prostheticknee technology, the Otto Bock C-Leg, enables amputees to walk withearly stance knee flexion and extension, and the state-of-the-art inankle-foot systems (such as the Össur Flex-Foot) allow for anklecontrolled plantar-flexion and dorsi-flexion. Although these systemsrestore a high level of functionality to leg amputees, they nonethelessfail to restore normal levels of ankle powered plantar-flexion, amovement considered important not only for biological realism but alsofor walking economy. In FIG. 2, ankle power data are shown for tennormal subjects walking at four walking speeds from slow (½ m/sec) tofast (1.8 m/sec). As walking speed increases, both positive mechanicalwork and peak mechanical power output increase dramatically. Manyankle-foot systems, most notably the Flex-Foot, employ springs thatstore and release energy during each walking step. Although some powerplantar-flexion is possible with these elastic systems, normalbiological levels are not possible. In addition to power limitations,the flex-foot also does not change stiffness in response todisturbances. The human ankle-foot system has been observed to changestiffness in response to forward speed variation and groundirregularities. In FIG. 3, data are shown for a normal subject walkingat three speeds, showing that as speed increases ankle stiffness duringcontrolled plantar-flexion increases.

Artificial legs with a mechanical impedance that can be modeled as aspring in parallel with a damper are known in the art. Some prostheseswith non-linear spring rates or variable damping rates are also known inthe art. Unfortunately, any simple linear or non-linear spring actioncannot adequately mimic a natural limb that puts out positive powerduring part of the gait cycle. A simple non-linear spring function ismonotonic, and the force vs. displacement function is the same whileloading the spring as while unloading the spring. It is an object of thepresent invention to provide actively electronically controlledprosthetic limbs which improve significantly on the performance ofartificial legs known in the art, and which require minimal power frombatteries and the like. It is a further object of the present inventionto provide advanced electronically-controlled artificial legs whichstill function reasonably well should the active control function fail(for instance due to power to the electronics of the limb being lost).Still further, it is an object of the present invention to provideartificial legs capable of delivering power at places in the gait cyclewhere a normal biological ankle delivers power. And finally, it is anobject of the present invention to provide prosthetic legs with acontrolled mechanical impedance and the ability to deliver power, whileminimizing the inertial moment of the limb about the point where itattaches to the residual biological limb.

During use, biological limbs can be modeled as a variable spring-ratespring in parallel with a variable damping-rate damper in parallel witha variable-power-output forcing function (as shown in FIG. 4a ). In someactivities, natural human limbs act mostly as spring-dampercombinations. One example of such an activity is a slow walk. Whenwalking slowly, a person's lower legs (foot and ankle system) act mostlyas a system of springs and dampers. As walking speed increases, theenergy-per-step put out by the muscles in the lower leg increases. Thisis supported by the data in FIG. 2.

Muscle tissue can be controlled through nerve impulses to providevariable spring rate, variable damping rate, and variable forcingfunction. It is an objective of the present invention to better emulatethe wide range of controllability of damping rate, spring rate, andforcing function provided by human muscles, and in some cases to providecombination of these functions which are outside the range of naturalmuscles.

SUMMARY OF THE INVENTION

There are two major classes of embodiments of the present invention. Thefirst major class provides for actively controlled passive mechanicalparameters (actively controlled spring rate and damping rate). Thismajor class of embodiments will be referred to as variable-stiffnessembodiments. Three sub-classes of variable-stiffness embodiments aredisclosed:

-   -   1) Multiple parallel interlockable springs.    -   2) Variable mechanical advantage.    -   3) Pressure-variable pneumatics.

The second major class of embodiments of the present invention allowsfor the controlled storage and release of mechanical energy within agait cycle according to any arbitrary function, including functions notavailable through simple nonlinear springs. Within this second majorclass of embodiments, energy can be stored and released at rates whichare variable under active control. Thus for a given joint, the force vs.displacement function is not constrained to be monotonic orsingle-valued. Within this class of embodiments, energy (from eithermuscle or a separate on-board power source) can be stored and releasedalong arbitrarily defined functions of joint angular or lineardisplacement, force, etc. This major subclass of embodiments shall bereferred to herein as energy transfer embodiments. Two sub-classes ofenergy transfer embodiments are disclosed:

-   -   1) Bi-articular embodiments (which transfer energy from a        proximal joint to a distal joint to mimic the presence of a        missing joint).    -   2) Catapult embodiments (which store energy from a power source        over one span of time and release it over another span of time        to aid locomotion).

The present invention makes possible prostheses that have mechanicalimpedance components (damping and spring rate) and power outputcomponents that are actively controllable as functions of jointposition, angular velocity, and phase of gait. When used in a prostheticleg, the present invention makes possible control of mechanicalparameters as a function of how fast the user is walking or running, andas a function of where within a particular step the prosthetic leg isoperating.

It is often necessary to apply positive mechanical power in runningshoes or in orthotic and prosthetic (O&P) leg joints to increaselocomotory speed, to jump higher, or to produce a more natural walkingor running gait. For example, when walking at moderate to high speeds,the ankle generates mechanical power to propel the lower leg upwards andforwards during swing phase initiation. In FIG. 2, data are shown forten normal subjects showing that the ankle delivers more energy during asingle step than it absorbs, especially for moderate to fast walkingspeeds.

Two catapult embodiments of the present invention are described in whichelastic strain energy is stored during a walking, running or jumpingphase and later used to power joint movements. In a first embodiment,catapult systems are described in which storage and release of storedelastic energy occurs without delay. In a second embodiment, elasticstrain energy is stored and held for some time period before release. Ineach Embodiment, mechanism architecture, sensing and control systems aredescribed for shoe and O&P leg devices. Although just a few devices aredescribed herein, it is to be understood that the principles could beused for a wide variety of applications within the fields ofhuman-machine systems or legged robots. Examples of these first andsecond catapult embodiments are shown in FIGS. 4 through 6.

One bi-articular embodiment of the invention described herein comprisesa system of knee-ankle springs and clutches that afford a transfer ofenergy from hip muscle extensor work to artificial ankle work to powerlate stance plantar-flexion. Since the energy for ankle plantar-flexionoriginates from muscle activity about the hip, a motor and power supplyneed not be placed at the ankle, lowering the total mass of theknee-ankle prosthesis and consequently the metabolic cost associatedwith accelerating the legs in walking. Examples of these embodiments areshown in FIGS. 7 and 8.

Several variable-stiffness embodiments are described herein in whichvariable spring-rate structures are constructed by varying the length ofa moment arm which attaches to a spring element about a pivot axis, thusproviding a variable rotational spring rate about the pivot axis.Examples of such embodiments are depicted in FIGS. 9 through 11. In apreferred embodiment, variations in the length of the moment arm aremade under microprocessor control at times of zero load, to minimizepower consumed in the active control system.

Variable-stiffness embodiments of the present invention employingmultiple interlockable parallel spring elements are depicted in FIGS. 12through 14. In FIGS. 12a and 12b , multiple parallel elastic leaf springelements undergo paired interlocking at pre-set joint flexures or undermicroprocessor control. This embodiment makes possible arbitrarypiecewise-linear approximations to non-linear spring functions (such asfunction 624 in FIG. 12d ). A pneumatic embodiment which can beconfigured to behave similarly to the leaf spring embodiments shown inFIGS. 12a and 12b is shown in FIG. 13. In the pneumatic embodiment ofFIG. 13, valves are electronically closed to effectively increase thenumber of pneumatic springs in parallel.

The multiple parallel spring elements in FIGS. 12a, 12b , and FIG. 13could equivalently be replaced by other types of spring elements, suchas coil springs, torsion bars, elastomeric blocks, etc.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Depiction of stages of a gait cycle, including controlledplantar-flexion, controlled dorsi-flexion, and powered plantar-flexion.

FIG. 2: Data from ten normal subjects are plotted showing mechanicalpower output versus percent gait cycle in walking. Both zero and onehundred percent gait cycle correspond to heel strike of the same foot

FIG. 3: Data for one subject, showing normal biological ankle functionduring the controlled plantar-flexion phase of walking.

FIG. 4a : Basic catapult embodiment of the present invention,represented in terms of a lumped-parameter model.

FIG. 4b : Force-displacement graph where darkened area represents extrastored energy (used in walking/running) put into catapult system byforce actuator while prosthetic foot is off the ground.

FIG. 4c : Side view of simplified prosthetic mechanism designed toprovide powered plantar-flexion.

FIG. 4d : Front view of simplified prosthetic mechanism designed toprovide powered plantar-flexion.

FIG. 5a : Catapult foot prosthesis or shoe orthosis for walking,running, and jumping, shown in the equilibrium configuration.

FIG. 5b : Catapult foot prosthesis or shoe orthosis for walking,running, and jumping, shown in a compressed state.

FIG. 6a : Side view of catapult leg prosthesis for walking, running, andjumping, shown in the equilibrium state.

FIG. 6b : Side view of catapult leg prosthesis for walking, running, andjumping, shown in a compressed state.

FIG. 6c : Front view of catapult leg prosthesis for walking, running,and jumping.

FIG. 7: An external, bi-articular transfemoral prosthesis or orthosis isshown in a heel strike to toe-off walking sequence. The system comprisessprings and controllable clutches to transfer energy from hip muscularwork to ankle powered plantar-flexion work.

FIG. 8: An external, bi-articular transfemoral prosthesis or orthosis isshown in a heel strike to toe-off walking sequence. The system comprisespneumatic springs and controllable valves to transfer energy from hipmuscular work to ankle powered plantar-flexion work.

FIG. 9: Perpendicularly-variable-moment pivotal spring structure.

FIG. 10: Mechanical diagram of a low-profile prosthetic foot wherespring elements are actively controlled (positioned) to affect anklejoint stiffness.

FIG. 11: Variable-stiffness joint according to the present invention,utilizing variable mechanical advantage to produce variable spring rateand/or variable damping rate.

FIG. 12a : Multiply interlockable parallel leaf spring structure, shownin equilibrium position.

FIG. 12b : Multiply interlockable parallel leaf spring structure, shownin a stored-energy position.

FIG. 12c : End view of two dove-tailed slidably attached leaf springterminations with controllable interlock actuator.

FIG. 12d : Piecewise-linear approximation to nonlinear spring functionachieved by interlocking successive parallel leaf springs at variousangles, and smoothed nonlinear spring function achieved by interlockingsuccessive parallel leaf springs through coupling springs.

FIG. 12e : Nonlinear damping element coupling mechanism for couplingmultiple spring elements.

FIG. 13: Multiple-pneumatic-chamber variable spring rate and energytransfer system.

FIG. 14: Prosthetic ankle/foot utilizing multiple interlockable parallelleaf springs for ankle spring.

FIG. 15: Example prosthetic ankle/foot known in the art.

FIG. 16: Variable-stiffness pneumatic spring.

DETAILED DESCRIPTION

A powered-catapult embodiment of the present invention is shown in FIGS.4a-4d . FIG. 4a is a lumped-element model of a powered-catapultprosthetic. The mounted end 203 of the prosthesis attaches to the body,and the distal end 204 of the prosthesis interfaces to the environment(such as the ground for a leg prosthesis). Mounted end 203 is coupled todistal end 204 through spring 202, and through the series combination offorce actuator 205 and force sensor 201. In some embodiments,displacement sensor 206 may also be included in parallel with spring202. If the system is designed to operate in parallel with an existinglimb, the muscles of the existing limb are modeled by muscle 200.

A mechanical implementation of lumped-element diagram 4 a is shown inside view in FIG. 4c and in front view in FIG. 4d . In a preferredembodiment, during the portion of a gait cycle when the foot is not incontact with the ground, motor 205 turns spool 209 to wind on some oftension band 208, storing energy in spring 202. Force sensor 201 andwinding distance sensor 207 may be used in a control loop to control howmuch energy is stored in spring 202, and how rapidly this energy isstored. Once the desired energy has been stored, clutch 207 is actuatedto keep tension band 208 from unwinding and spring 202 from relaxinguntil the control system decides to release the stored energy. Theenergy stored in spring 202 during the swing phase of the gait cycle isrepresented by the dark area on the force vs. distance graph shown inFIG. 4 b.

During the powered plantar-flexion phase of the gait cycle, the controlsystem releases clutch 207, allowing the stored energy in spring 202 tobe released, imitating the powered plantar-flexion stage of a normalgait cycle. This release of energy mimics the pulse of power put out bya biological ankle during the powered plantar-flexion stage of a walkingor running gait cycle.

In an alternate embodiment, motor 205 may store energy in spring 202 atthe same time as the natural leg stores impact energy during the gaitcycle. This embodiment can be used to effectively implement one springrate during compression (such as the spring rate depicted by the linefrom the origin to point Kd in FIG. 4b ) and another spring rate duringrelease (such as the spring rate depicted by the line from the origin topoint Ks in FIG. 4b ).

In an alternate embodiment, FIG. 5 shows a prosthetic foot or shoeorthosis that stores both muscle energy and motor energy in springmechanism 300 during the gait cycle, for release during the poweredplantar-flexion stage of the walking gait cycle (toe-off propulsion).When walking on this type of catapult prosthesis or foot orthosis, aperson would experience a first (lower) spring rate (depicted by theline from the origin to point Kd in FIG. 4b ), and a second (higher)spring rate (depicted by the line from the origin to point Ks in FIG. 4b) when releasing energy from spring 300 during the poweredplantar-flexion phase of the gait cycle.

For catapult embodiments depicted in both FIG. 4 and in FIG. 5, part ofthe energy released during powered plantar-flexion came from leg muscleaction compressing springs 202 and 300, and part came from anelectromechanical actuator such as a motor. In a preferred embodiment ofthe present invention as depicted in FIG. 4, the majority of powerstored in spring mechanisms by electromechanical actuators occurs duringthe minimal-load portion of the walking/running gait cycle (swingphase), and the start of the energy-release phase (late stance phase) ofthe gait cycle may be time-delayed with respect to the swing phase whenmotor energy is stored.

FIG. 6 is another depiction of the catapult leg prosthesis of FIG. 4,also showing socket 400, which attaches to the residual biological limb.Although the leg prostheses shown in FIGS. 4 and 6 are below-the-kneeprostheses, the invention could also be employed in above-kneeprostheses.

Two bi-articular embodiments of the present invention are shown in FIGS.7 and 8. In a first embodiment (FIG. 7), a prosthesis (above or belowknee), robotic leg or full leg orthosis is shown having above-kneesegment (a), knee joint (b), ankle joint (c), posterior knee pivot (d),posterior clutch (e), posterior spring (f), posterior cord (g),knee-ankle transfer clutch (h), anterior pivot (i), anterior clutch (j),anterior spring (k), and anterior cord (l). Anterior spring (k)stretches and stores energy during early stance knee flexion (from 1 to3) and then releases that energy during early stance knee extension(from 3 to 5). Here spring (k) exerts zero force when the knee is fullyextended, and anterior clutch (j) is engaged or locked throughout earlystance knee flexion and extension (from 1 to 5). This stored energy,together with an applied extensor hip moment from either a robotic orbiological hip, result in an extensor moment at the knee, forcing theknee to extend and stretching posterior spring (f) (from 3 to 5). Thespring equilibrium length of posterior spring (f) is equal to theminimum distance from posterior knee pivot (d) to posterior clutch (e)(leg configuration 3 in FIG. 7). To achieve this spring equilibrium,posterior clutch (e) retracts posterior cord (g) as the distance fromposterior knee pivot (d) to posterior clutch (e) becomes smaller. Whenthis distance begins to increase in response to knee extension and ankledorsi-flexion (from 4 to 5), posterior clutch (e) engages, causingposterior spring (f) to stretch. When the ankle is maximallydorsi-flexed and the knee fully extended (leg configuration 5),posterior spring (f) becomes maximally stretched. When the leg assumesthis posture, knee-ankle transfer clutch changes from a disengaged stateto an engaged state. Engaging the knee-ankle clutch mechanically groundsspring (f) below the knee rotational axis, and consequently, all theenergy stored in spring (f) is transferred through the ankle to powerankle plantar-flexion (from 6 to 7). During late stance (from 5 to 6),the knee of the supporting leg begins to flex again in preparation forthe swing phase. For this late stance knee flexion, anterior clutch (j)is disengaged to allow the knee to freely flex without stretchinganterior spring (k).

It should be understood that the bi-articular knee-ankle invention ofembodiment I (FIG. 7) could assume many variations as would be obviousto those of ordinary skill in the art. For example, the system describedherein could act in parallel to additional ankle-foot springs and/or toan active or passive knee damper. Additionally, instead of mechanicallygrounding spring (f) distal to the knee axis to effectively transfer allthe stored energy through the ankle, the perpendicular distance from theline of spring force (f) to the knee's axis of rotation could go to zeroas the knee approaches full extension.

In a second embodiment (FIG. 8), a prosthesis (above or below knee),robotic leg or full leg orthosis is shown having a similar energytransfer from hip muscle extensors to artificial leg to power ankleplantar-flexion, accept energies are stored within pneumatic springsabout the knee and then transferred to the ankle via a fluid transfersystem. In this embodiment, the transfer of energy occurs without aphysical bi-articular spring such as posterior spring (f) in FIG. 7. Inthis embodiment, anterior pneumatic spring (j) compresses and storesenergy during early stance knee flexion (from 1 to 3). Here anteriorknee valve (k) is closed or locked throughout early stance knee flexionand extension (from 1 to 5). This stored energy, together with anapplied extensor hip moment from either a robotic or biological hip,result in an extensor moment at the knee, forcing the knee to extend andcompress posterior pneumatic spring (f) (from 3 to 5). It is importantto note that posterior knee valve (g) is open during early stance kneeflexion so that posterior pneumatic spring (f) exerts little force. Kneevalve (g) is then closed during knee extension so that energy is storedin the posterior pneumatic spring (f). When the ankle is maximallydorsi-flexed and the knee fully extended (leg configuration 5),posterior pneumatic spring (f) is maximally compressed. When the legassumes this posture, knee-ankle transfer valve changes from a closedstate to an open state, and anterior ankle valve (n) changes to a closedstate, allowing all the energy stored in spring (f) is be transferredthrough the ankle to power ankle plantar-flexion (from 6 to 7). Duringlate stance (from 5 to 6), the knee of the supporting leg begins to flexagain in preparation for the swing phase. For this late stance kneeflexion, anterior and posterior valves (g, k) are open to allow the kneeto freely flex without compressing anterior spring (j).

It should be understood that the bi-articular knee-ankle invention ofembodiment II (FIG. 8) could assume many variations as would be obviousto those of ordinary skill in the art. For example, the system describedherein could act in parallel to active or passive ankle-foot springsand/or to an active or passive knee damper. Additionally, the energy inposterior pneumatic spring (f) could be transferred to a temporaryholding chamber to be later released to the ankle during poweredplantar-flexion.

The mechanical system in FIG. 9 is a variable-mechanical-advantageembodiment of a variable-stiffness spring. Motors 500 and motor-drivenscrews 505 serve to change the moment of compression of bow spring 503about pivot point 504. This mechanism may be used to adjust springstiffness with minimal power under no-load conditions. It may also beused as an alternative way of storing energy in a spring which is underload, and thus may be used as a component of an immediate-releasecatapult system such as depicted in FIG. 5.

FIG. 10 depicts a low-profile prosthetic foot-ankle with top plate 1 andbottom plate 2, where spring elements are actively controlled(positioned) to affect ankle joint stiffness. This embodiment of thepresent invention is a variable-stiffness embodiment of the “variablemechanical advantage” sub-class. In this low-profile prosthetic anklejoint embodiment, side-to-side spring rates of the prosthetic ankle andfront-to-back spring rates of the prosthetic ankle are adjusted byvarying the distance of spring elements 4, 5, 6, and 7 from the centralpivot point 15 of the ankle joint. Spring top plates 13 and springbottom plates 12 of spring elements 4, 5, 6, and 7 slide in tracks 14,driven by position-adjusting motors 8, 9, 10, and 11. In a preferredembodiment, motors 8, 9, 10, and 11 only change the positions of springelements 4, 5, 6, and 7 when the ankle joint is under zero load (forinstance, during the part of the walking gait when the foot is not incontact with the ground). Adjustment of spring position under zero loadallows position adjustments to be done with minimal energy. Thisembodiment offers independent inversion/eversion stiffness control aswell as independent plantar-flexion and dorsi-flexion control.

A variable stiffness ankle-foot prosthesis embodiment according to thepresent invention is shown in FIG. 11. Constant-rate spring or dampingelement 1700 fixedly attached at one end and movably attached at theother end. Attachment point 1701 may be moved in and out with respect tothe effective pivot point of the ankle joint. If element 1700 is adamping element, this configuration provides a variable damping anklejoint. If element 1700 is a spring element, this configuration providesa variable spring rate ankle joint. FIGS. 9, 10 and 11 demonstrate how aconstant element can be transformed into a variable element according tothe present invention, by varying mechanical advantage. In non-catapultpreferred embodiments of the present invention, the variation inmechanical advantage takes place such that the motion used to vary themechanical advantage takes place substantially perpendicular to theforce the element being moved is under, thus minimizing the work neededto vary the mechanical advantage.

FIGS. 12a and 12b depict a multiple-parallel-leaf-spring embodiment of avariable mechanical impedance according to the present invention. Leafsprings 600 are bound together and bound tightly to attaching bracket602 at one end by bolt 801. At the other end, leaf springs terminate inslidably interlocking blocks 603, which may be locked togetherdynamically in pairs by interlocking plates 605. Each interlocking plate605 is permanently bonded to one leaf spring terminator block 603 atsurface interface 606, and controllably bindable to a second leaf springterminator block 604 at a second interface 607, by binding actuator 608.Binding actuator 608 may bind surface interface 607 by any number ofmeans such as mechanical clamp, pin-in-socket, magnetic clamp, etc.Adjacent leaf spring terminator blocks are slidably attached by dovetailslides or the like. The structure shown in FIGS. 12a-c can be used toimplement a piecewise-linear spring function such as function 604depicted in FIG. 12d , by engaging successive interlocks 605 atpre-determined points in spring flexure, and disengaging at like points.

In a preferred embodiment, the slope discontinuities in function 604 maybe “smoothed” by coupling successive leaf springs through couplingsprings. In FIG. 12d , stop plate 619 is affixed to leaf springtermination 620, and coupling spring 621 is mounted to leaf springtermination 618 through coupling spring mount 622. Leaf springtermination 620 is free to slide with respect to leaf spring termination618 until coupling spring 621 and stop plate 619 come in contact.Coupling spring 621 acts to smooth the transition from the uncoupledstiffness of two leaf springs to the coupled stiffness of two leafsprings, resulting in smoothed force-displacement function 625 in FIG.12 d.

In a preferred embodiment, coupling spring 621 is itself a stiff,nonlinear spring. In another preferred embodiment, coupling spring 621may have actively controllable stiffness, and may be made according toany of variable-stiffness spring embodiments of the present invention.

FIG. 12e depicts a non-linear dissipative coupling mechanism forcoupling pairs of spring elements in a multiple-parallel-element spring.Mechanical mounts 609 and 610 affix to a pair of spring elements to becoupled. In a preferred embodiment, one of 609 and 610 is permanentlyaffixed and the other of 609 and 610 is controllably affixed through amechanism such as 608 described above. Piston 611 is coupled to mount609 through rod 612 which passes through seal 614. Thus piston 611 maymove back and forth in chamber 615 along the axis of rod 612. Chamber615 is preferably filled with viscose or thixotropic substance 616. Aviscose substance can be used in chamber 616 to provide a mechanicalcoupling force proportional to the square of the differential velocitybetween mounts 609 and 610. A thixotropic substance (such as a mixtureof corn starch and water) can be used to provide an even more nonlinearrelationship between coupling force and the differential velocitybetween coupling plates 609 and 610. Alternately, an electronicallycontrolled variable damping element may be used in series with forcesensor 617 between mounts 609 and 610, to provide an arbitrarynon-linear dissipative coupling.

Utilizing a nonlinear dissipative coupling between pairs of elements ina multiple-parallel-element spring allows joint spring rates in aprosthetic limb which are a function of velocity. Thus, a joint springrate can automatically become stiffer when running than it is whilewalking.

In one preferred embodiment, chamber 615 is rigidly mounted to mount610. In another preferred embodiment, chamber 615 is mounted to mount610 through coupling spring 623. In a preferred embodiment, couplingspring 623 may be an actively-controlled variable stiffness springaccording to the present invention.

FIG. 13 depicts a multiple-couplable-parallel element pneumaticembodiment of the present invention. Multiple parallel pneumaticchambers 900 couple mounting plates 908 and 909. Pneumatic hoses 902connect chambers 900 to a common chamber 901 through individuallyactuatable valves 903. Spring stiffness between plates 908 and 909 ismaximized when all valves 903 are closed, and minimized when all valves903 are open. Additional pneumatic element 905 may be added to transferpower from one prosthetic joint to another.

In an immediate-energy-transfer embodiment of the present inventionaccording to FIG. 13, valves 904 and 906 may be timed to actuate insequence with valves 903 to transfer power directly from chamber 905 tochambers 900. In a delayed-energy-transfer embodiment of the presentinvention according to FIG. 13, energy may be transferred from chamber905 to chambers 900 or vice versa in a delayed manner, by chambers 900or chamber 905 first pressurizing chamber 901, then isolating chamber901 by closing valves 903 and 904 for some period of time, thentransferring the energy stored in chamber 901 to chambers 900 or 905 byopening the appropriate valves.

FIG. 15a depicts a prosthetic ankle-foot system known in the art. Anklespring 1500 is affixed to foot-plate 1501. One variable-stiffnessembodiment of the present invention shown in FIG. 15 uses amultiple-parallelly-interlockable-leaf-spring structure such as thatshown in FIG. 12 in place of ankle spring 1500.Multiple-parallelly-interlockable-leaf-spring 1600 allows for differentspring rates in forward and backward bending, allowing separatelycontrollable rates of controlled plantar-flexion and controlleddorsi-flexion.

In one embodiment of the present invention (shown in FIG. 15b ), anklespring 1500 is split into inner ankle spring 1500 a, and outer anklespring 1500 b, and heel spring 1501 is split rearward of attachmentpoint AP into inner heel spring 1501 a and outer heel spring 1501 b. Ina preferred embodiment, ankle springs 1500 a and 1500 b and heel springs1501 a and 1501 b each comprise actively-variable multi-leaf springssuch as ankle spring 1600 in FIG. 14. Having separate inner and outervariable-stiffness ankle springs allows for active control ofside-to-side stiffness of the prosthetic ankle joint. Having separateinner and outer variable-stiffness heel springs allows for activecontrol medio-lateral ankle stiffness.

A pneumatic embodiment of a variable-stiffness spring for a prosthesisis shown in FIG. 16. Male segment 702 comprises one end of the overallvariable-stiffness spring, and female segment 701 comprises the otherend. Control electronics 710 are contained in the upper end of malesegment 710. Intake valve 715 is actuatable to allow air to enterpressure chamber 708 through air intake channel 716 when pressurechamber 708 is below atmospheric pressure (or an external pump may beused to allow air to enter even when chamber 708 is above atmosphericpressure). Air pressure sensor 709 senses the pressure in pressurechamber 708. Pressure chamber 708 is coupled to second pressure chamber703 through valve 711. The air in pressure chamber 703 acts as apneumatic spring in parallel with spring 704. Motor 705 turns ball screw707 to move piston 706 back and forth to control the volume of pressurechamber 708. Pressure in pressure chamber 703 may be lowered to adesired value by opening valve 703 for a controlled period of time,allowing air to escape through pressure release channel 714.

In one mode of operation, valve 711 is open and pressure chambers 708and 703 combine to form a single pressure chamber. In this mode,movement of piston 706 directly controls the overall pressure chambervolume, and thus the overall pneumatic spring rate. In another mode ofoperation, valve 711 is closed, and valve 706 may be opened and piston706 may withdrawn to add air to the system.

In a preferred embodiment of a variable-stiffness leg prosthesisaccording to the present invention is implemented through the pneumaticsystem of FIG. 16, motion of piston 706 occurs under minimal load, suchas during the phase of gait when the foot is off the ground, or when theuser is standing still.

The pneumatic system shown in FIG. 16 may also be used to implementImmediate-release or delayed-release catapult embodiments of the presentinvention. An immediate-release catapult may be implemented by openingvalve 711, and using motor 705 to add power (for instance, during thepowered plantar-flexion phase of gait) as the power is needed. In adelayed-release catapult embodiment of the present invention, valves 715and 711 are closed while motor 705 moves piston 706 to pressurizechamber 708, and then energy stored in chamber 708 is rapidly releasedduring a phase of gait to produce the same effect as poweredplantar-flexion.

In a preferred embodiment of the present invention, a pneumaticprosthetic leg element according to FIG. 16 is combined with themultiple controllably-couplable parallel leaf spring prostheticankle-foot of FIG. 15 to provide a prosthetic limb which providespowered plantar-flexion, controllable compressional leg springstiffness, and controllable ankle stiffness during controlledplantar-flexion and controlled dorsi-flexion.

The foregoing discussion should be understood as illustrative and shouldnot be considered to be limiting in any sense. While this invention hasbeen particularly shown and described with references to preferredembodiments thereof, it will be understood by those skilled in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope of the invention as defined by theclaims.

What is claimed is:
 1. A bi-articular prosthetic or orthotic legcomprising: an above-knee segment configured to attach to a biologicallimb of a human body above a knee of the human body; a support memberconnected at its upper end to said above-knee segment, said supportmember configured to rotate with respect to said above-knee segment todefine a knee joint; a foot member attached to a lower end of thesupport member, the foot member including a rearwardly extending heelportion and a forwardly extending toe portion to make periodic bearingcontact with ground during walking, running or jumping gait cycle of ahuman body, said foot member configured to rotate relative to saidsupport member to define an ankle joint; first and second anteriorpneumatic springs; the first anterior pneumatic spring connected at itsupper end to the above-knee segment and connected at its lower end to amid-portion of the support member, the first anterior spring displacedfrom the knee joint and configured to store energy when said supportmember rotates about said knee joint to move said foot member rearwardlywith respect to said above-knee segment and configured to release energyto rotate said support member about said knee joint; the second anteriorpneumatic spring connected at its upper end to said mid-portion of thesupport member and connected at its lower end to said foot member; ananterior knee valve operably connected to said first anterior pneumaticspring; an anterior ankle valve operably connected to said secondanterior pneumatic spring; a posterior pneumatic spring connected at itsupper end to the above-knee segment and connected at its lower end tosaid mid-portion of the support member, the posterior spring displacedfrom the knee joint and configured to store energy when said supportmember is rotated in an extension movement about said knee joint andconfigured to store energy to rotate said foot member about said anklejoint to increase bearing force applied to the ground by said toeportion during powered ankle plantar-flexion and to impart lifting forceagainst the ground; and a posterior knee valve operably connected tosaid posterior pneumatic spring; wherein each of the springs isconfigured to store and release energy via a fluid transfer systemcontrolled by the anterior knee valve, the posterior knee valve, and theanterior ankle valve, wherein the anterior knee valve is closed orlocked throughout early stance knee flexion and extension, wherein theposterior knee valve is open during early stance knee flexion, whereinwhen the ankle is dorsi-flexed and the knee extended: the posteriorpneumatic spring is compressed, the posterior knee valve changes from aclosed state to an open state, and the anterior ankle valve changes to aclosed state, allowing energy stored in the posterior pneumatic springto be transferred through the ankle during powered ankleplantar-flexion.
 2. The prosthetic or orthotic leg of claim 1, whereinthe first anterior pneumatic spring is configured to store energy whensaid support member rotates about said knee joint to move said footmember rearwardly with respect to said knee joint during an early stanceknee flexion stage of a gait cycle that follows heel-strike when saidheel portion of said foot member first contacts the ground and is toprovide shock absorption.
 3. The prosthetic or orthotic legs of claim 2,wherein energy released by the first anterior pneumatic spring istransferred to the posterior spring as said support member rotates aboutsaid knee joint during a knee extension movement.
 4. The prosthetic ororthotic leg of claim 1, wherein the posterior pneumatic spring also isconfigured to store energy during a dorsi-flexion stage of a gait cyclewhen said support member rotates forwardly about said ankle joint assaid foot member remains in contact with the ground.
 5. The prostheticor orthotic leg of claim 4, wherein energy released by the firstanterior pneumatic spring is transferred to the posterior pneumaticspring as said support member rotates about said knee joint during aknee extension movement.
 6. The prosthetic or orthotic leg of claim 5,wherein at least one of the anterior pneumatic springs is connected inseries with an anterior clutch to engage said at least one of theanterior pneumatic springs at select engagement times during a walkingor running gait cycle.
 7. The prosthetic or orthotic leg of claim 5,wherein the first anterior pneumatic spring compresses and stores energyduring an early stance knee flexion.
 8. The prosthetic or orthotic legof claim 1, wherein energy released by the first anterior pneumaticspring is transferred to the posterior pneumatic spring at apredetermined time during a gait cycle.
 9. The prosthetic or orthoticleg of claim 1, wherein energy released by the first anterior pneumaticspring is transferred to the posterior pneumatic spring while saidsupport member rotates about said knee joint in a knee extensionmovement.
 10. The prosthetic or orthotic leg of claim 1, wherein theposterior pneumatic spring is configured to release energy during apowered plantar-flexion stage of a gait cycle when the said toe portionof said foot member presses against the ground and raises said heelportion from the ground delivering power to the walking step.
 11. Theprosthetic or orthotic leg of claim 1, wherein at least one of theanterior pneumatic springs is connected in series with an anteriorclutch to engage said at least one of the anterior pneumatic springs atselect engagement times during a gait cycle.